REGN-EB3 and mAb114 were developed using antibodies harvested from survivors of Ebola, which has killed more than 1,800 people in DR Congo in the past year.

Two other treatments, called ZMapp and Remdesivir, have been dropped from trials as they were found to be less effective.

The trial, conducted by an international research group coordinated by the World Health Organization (WHO), began in November last year.

Since then, four experimental drugs have been tested on around 700 patients, with the preliminary results from the first 499 now known.

Of the patients given the two more effective drugs, 29% on REGN-EB3 and 34% on mAb114 died, NIAID said.

In contrast, 49% on ZMapp and 53% on Remdesivir died in the study, the agency said.

The survival rate among patients with low levels of the virus in their blood was as high as 94% when they were given REGN-EB3 and 89% when on mAb114, the agency said.

The findings mean health authorities can “stress to people that more than 90% of people survive” if they are treated early, said Sabue Mulangu, an infectious-disease researcher who worked on the trial.